To get the whole stage in and close ups of the performers from the same stand point I reckon that you would need something like a 5x zoom and they just don't exist in f2.8 at any price, let alone for £300.
To reduce the need to move about so much you might look at something wide angle to compliment your 50mm but even then finding something within your budget that opens up to f2.8 is going to be a challenge. The closest I think you'll come to it is the Tokina AF 11-16mm f/2.8 AT-X Pro which gets good reviews
<<<HERE>>> but even on ebay you'll be looking at over £400.
The other alternative, assuming you've got an 18-55 standard lens to use for your wide angle shots, get something a bit longer for the close up eg the Canon 85mm f1.8 which would just about be in your budget